1. Field of the Invention
The present invention relates to a method for post-cleaning finish drying to be conducted in a final step subsequent to cleaning of industrial parts, in which finish drying of industrial parts, e.g., optical and molded parts composed of glass or plastic, metallic parts, ceramic parts and electronic parts, upon cleaning can be performed without causing the parts to have stain and residue, so that the industrial parts having undergone the finish drying can be directly fed to surface treatment, e.g., formation of a vacuum deposition film on the surface of the parts. In particular, the present invention is concerned with the method in which finish drying can be accomplished without the use of freon, the reduction of the use thereof being worldwide demanded in recent years.
2. Discussion of Related Art
Generally, for industrial parts, such as optical, molded and electronic parts, finish drying is performed after precision cleaning thereof for defatting. This post-cleaning finish drying has been performed by the vapor drying using 1,1,2-trichloro-1,2,2-trifluoroethane (freon 113). The freon 113 has predominantly been used because it is incombustible, has low toxicity to organisms, can rapidly be dried and exhibits selective dissolving power (fats and oils are effectively dissolved while polymeric materials, such as plastic and rubber, are not corroded).
However, freon 113 (and other perhaloethanes) are so chemically stable that the life thereof in the troposphere is long. Thus, freon 113 is diffused into the stratosphere, where it is decomposed by sunbeams to produce halogen radicals. The halogen radicals incur chain reaction with ozone to thereby destroy the ozonosphere. Therefore, the reduction of the use of freon 113 is strongly demanded. In accordance with the demand for the reduction of the use of freon 113 for the protection of the ozonosphere, proposals have been made to carry out finish drying by the use of a large variety of solvent mixtures and azeotropic compositions. For example, Japanese Unexamined Patent Application Publication No. 318094/1989 discloses a solvent composed of a mixture of freon 113, isopropyl alcohol and methyl ethyl ketone. Japanese Unexamined Patent Application Publication No. 289693/1990 discloses azeotropic compositions comprising dichlorotetrafluoropropane (freon 234) and an aliphatic lower alcohol, such as ethanol. Further, the finish drying using the vapor of isopropyl alcohol (IPA) is disclosed.
However, the mixture disclosed in Japanese Unexamined Patent Application Publication No. 318094/1989 contains freon 113 as an essential ingredient. Therefore, the reduction of the use of freon 113 is limited. On the other hand, the azeotropic compositions disclosed in Japanese Unexamined Patent Application Publication No. 289693/1990 contain freon 234. From the viewpoint of destruction of the ozonosphere, freon 234 is less powerful than freon 113. However, freon 234 cannot completely be free from destruction of the ozonosphere. Further, the use of the vapor of IPA is not practical because it is likely to have influence from water, etc., it is inflammable to thereby have the danger of inflammation, and there is a problem of degrading plastics, etc.